Mass Exploitation of Exchange Server Zero-Day CVEs: What You Need to Know

On March 2, 2021, the Microsoft Threat Intelligence Center (MSTIC) released details on an active state-sponsored threat campaign exploiting four zero-day vulnerabilities in on-premises instances of Microsoft Exchange Server. MSTIC attributes this campaign to HAFNIUM, a group “assessed to be state-sponsored and operating out of China.”

Rapid7 detection and response teams have also observed increased threat activity against Microsoft Exchange Server since Feb. 27, 2021, and can confirm ongoing mass exploitation of vulnerable Exchange instances. Microsoft Exchange customers should apply the latest updates on an emergency basis and take immediate steps to harden their Exchange instances. We strongly recommend that organizations monitor closely for suspicious activity and indicators of compromise (IOCs) stemming from this campaign. Rapid7 has a comprehensive list of IOCs available here.

The actively exploited zero-day vulnerabilities disclosed in the MSTIC announcement as part of the HAFNIUM-attributed threat campaign are:

• CVE-2021-26855 is a server-side request forgery (SSRF) vulnerability in Exchange that allows an attacker to send arbitrary HTTP requests and authenticate as the Exchange server.
• CVE-2021-26857 is an insecure deserialization vulnerability in the Unified Messaging service. Insecure deserialization is where untrusted user-controllable data is deserialized by a program. Exploiting this vulnerability gives an attacker the ability to run code as SYSTEM on the Exchange server. This requires administrator permission or another vulnerability to exploit.
• CVE-2021-26858 is a post-authentication arbitrary file write vulnerability in Exchange. If an attacker could authenticate with the Exchange server, they could use this vulnerability to write a file to any path on the server. They could authenticate by exploiting the CVE-2021-26855 SSRF vulnerability or by compromising a legitimate admin’s credentials.
• CVE-2021-27065 is a post-authentication arbitrary file write vulnerability in Exchange. An attacker who can authenticate with the Exchange server can use this vulnerability to write a file to any path on the server. They could authenticate by exploiting the CVE-2021-26855 SSRF vulnerability or by compromising a legitimate admin’s credentials.

Also included in the out-of-band update were three additional remote code execution vulnerabilities in Microsoft Exchange. These additional vulnerabilities are not known to be part of the HAFNIUM-attributed threat campaign but should be remediated with the same urgency nonetheless:

• CVE-2021-26412 (CVSS:3.0 9.1 / 8.2)
• CVE-2021-26854 (CVSS:3.0 6.6 / 5.8)
• CVE-2021-27078 (CVSS:3.0 9.1 / 8.2)

Microsoft has released out-of-band patches for all seven vulnerabilities as of March 2, 2021. Security updates are available for the following specific versions of Exchange:

• Exchange Server 2010 (for Service Pack 3—this is a Defense in Depth update)
• Exchange Server 2013 (CU 23)
• Exchange Server 2016 (CU 19, CU 18)
• Exchange Server 2019 (CU 8, CU 7)

Exchange Online is not affected.

For Rapid7 customers

InsightVM and Nexpose customers can assess their exposure to these vulnerabilities with authenticated vulnerability checks. Customers will need to perform a console restart after consuming the content update in order to scan for these vulnerabilities.

InsightIDR will generate an alert if suspicious activity is detected in your environment. The Insight Agent must be installed on Exchange Servers to detect the attacker behaviors observed as part of this attack. If you have not already done so, install the Insight Agent on your Exchange Servers.

For individual vulnerability analysis, see AttackerKB.